A large number of multi-frequency antennas used in portable information terminals have been proposed. For example, PTL 1 discloses a multi-frequency antenna having a pair of antenna elements that radiate radio waves with frequencies f1 and f2 different from each other (f2>f1) and a feed line that feeds these antenna elements. In this multi-frequency antenna, a tip-opened line that grounds the feed line in a high-frequency manner at the higher frequency f2 is provided between the pair of antenna elements. According to this multi-frequency antenna, excitation of a predetermined frequency by the antenna element of another frequency is prevented and the desired characteristics of the directivity in the horizontal plane can be achieved.
Furthermore, the following dual-band antenna is disclosed in PTL 2. Specifically, a pair of antenna elements of λa (wavelength of operating frequency)/4 are disposed on the same straight line along the vertical direction to form a dipole antenna around λa. In addition, the lower antenna element is formed into a strip shape and it is employed as a ground plate of the upper antenna element to form an inverted-F antenna around λb (wavelength shorter than λa). The operating frequency band corresponding to λa is 900 to 930 [MHz] and the operating frequency band corresponding to λb is set to 1.85 to 1.99 [GHz].
In this dual-band antenna, a coaxial cable is connected to each of the antenna element of λa and the antenna element of λb and therefore radio waves of λa and radio waves of λb are transmitted and received as radio-frequency signals independent of each other. Furthermore, because the lower dipole antenna element is used also as the ground plate, the number of constituent components is reduced. Moreover, a trap that resonates at λb is provided close to the connecting point between the core line of the coaxial cable that feeds the radio-frequency signal of λb and the antenna element of λb to prevent the radio-frequency signal of λb from leaking into the radio-frequency circuit of λa. This allows transmission/reception of radio waves of λa by the dipole antenna and transmission/reception of radio waves of λb by the inverted-F antenna to be carried out independently of each other.